Removal Efficiency Of Total Cr Research Articles

Adsorption and Reduction of Aqueous Cr by FeS-Modified Fe-Al Layered Double Hydroxide

To remedy the widespread chromium (Cr) pollution in the environment, this study mainly used the ultrasonic-assisted co-precipitation and precipitation methods to prepare FeS-modified Fe-Al-layered double hydroxide (FeS/LDH) composite material. The experimental results showed that FeS/LDH has higher removal efficiency of Cr in aqueous solution and stronger anti-interference ability than unmodified LDH. Under the same reaction conditions, the removal efficiency of total Cr(Cr(T)) using LDH was 34.85%, and the removal efficiency of Cr(VI) was 46.76%. For FeS/LDH, the removal efficiency of Cr(T) and Cr(VI) reached 99.57% and 100%, respectively. The restoration of Cr(T) and Cr(VI) by FeS/LDH satisfied the Langmuir adsorption isotherm. The maximum adsorption capacity of Cr(T) and Cr(VI) achieved 102.9 mg/g and 147.7 mg/g. The efficient removal of Cr by FeS/LDH was mainly based on the triple synergistic effect of anion exchange between Cr(VI) and interlayer anions, redox of Cr(VI) with Fe2+ and S2−, and co-precipitation of Fe3+ and Cr3+.

Cr(VI) bioremediation by active algal-bacterial aerobic granular sludge: Importance of microbial viability, contribution of microalgae and fractionation of loaded Cr

In this study, chromium (Cr) was used as an example of the most toxic heavy metals that threaten human health, and Cr(VI) bioremediation was implemented by using a new type of aerobic granular sludge (AGS), i.e., algal-bacterial AGS. Results showed that the total Cr removal efficiency by active algal-bacterial AGS was 85.1 ± 0.6% after 6 h biosorption at pH 6 and room temperature, which could be further improved to 93.8 ± 0.4% with external electron donor (glucose) supply. However, inactivation dramatically decreased the total Cr removal efficiency to 29.6 ± 3.5%, and no effect was noticed when external electron donor was provided. With an antibiotic (levofloxacin) or metabolic inhibitor (NaN3) addition, the total Cr removal efficiency of bacterial AGS was inhibited by 16.0% or 10.1%, but this efficiency was maintained in the case of algal-bacterial AGS. Analysis of extracellular polymeric substances (EPS) composition revealed that under Cr(VI) exposure, more loosely bound EPS were secreted by algal-bacterial AGS, favoring Cr(VI) reduction. Results from chemical fractionation indicated that 90.5 ± 4.2% of the loaded Cr on algal-bacterial AGS was in an immobile form, reflecting the low environmental risk of Cr-loaded algal-bacterial AGS after biosorption of hazardous heavy metals from wastewater.

A high-efficiency process for the separation of chromium and aluminum from waste aluminum sludge with a high chromium content using a combined oxidation and dispersion process

Waste aluminum sludge (WAS) contains an abundance of Cr(III), which restricts the potential for recycling of Al resources in WAS. The oxidation process is a potentially effective method for the separation of Cr and Al from solid waste. However, many challenges restrict the application of oxidation treatments for WAS, including their low oxidative efficiency and their inability to handle sludge with high Cr concentration. This study proposes a highly efficient combined oxidative and dispersive process for the separation of Al and Cr from high Cr(III) content WAS. The effects of varying solid–liquid mass ratios and Ca(OCl)2 concentrations in the oxidation process and Na2SO4 concentrations in the dispersion process were investigated. Using single factor experiments, the optimal conditions were found to be a solid–liquid mass ratio of 1:14 and Ca(OCl)2 dosage of 280 mg/g dry WAS in the oxidative process, allowing the Cr(III) oxidative efficiency to reach 93.83%. After treatment using the combined processes, the total Cr removal efficiency reached 95.54% and the aluminum content exhibited almost no loss. Characterization by FTIR, XRD, and SEM-EDS indicated that Ca(OCl)2 destroyed the colloidal structure of WAS, resulting in the release of Cr(III) which could subsequently be oxidized to soluble Cr(VI) by Ca(OCl)2. In the dispersion process, Na2SO4 destroyed the colloidal stability of WAS, resulting in the release of Cr(VI) and improving the Cr removal efficiency. The combined processes were found to have a significantly synergistic effect, indicating that this method is suitable for the separation and collection of Cr and Al from WAS. Furthermore, the economic analysis showed that the combined processes have cost advantage over traditional treatment.

Electrochemical Cr(VI) removal from aqueous media using titanium as anode: Simultaneous indirect electrochemical reduction of Cr(VI) and in-situ precipitation of Cr(III)

In this work, a novel method for complete Cr(Ⅵ) removal was achieved in a single-chamber cell with titanium (Ti) as anode via simultaneous indirect electro-reduction of Cr(Ⅵ) and in-situ precipitation of Cr(Ⅲ). The Cr(Ⅵ) and total Cr removal, and electric energy consumption were optimized as a function of electrochemical reactor, current density, initial Cr(Ⅵ) and chloride (Cl−) concentration, and initial solution pH. The maximum Cr(Ⅵ) and total Cr removal efficiency reached 80.5 and 79.4% respectively within 12 h at current density of 10 mA cm−2 as initial Cr(Ⅵ) concentration was 0.078 mM. Decreasing the initial solution pH was beneficial to Cr(Ⅵ) reduction, but Cr(Ⅲ) precipitation was inhibited, resulting in the poor total Cr removal. The suitable Cl− concentration guaranteed sufficient reducing agents (Ti3+ and Ti2+) for Cr(Ⅵ) removal. The reaction mechanism demonstrated that Ti anode could be corroded to produce Ti3+ and Ti2+, which provided the electrons for reduction of Cr(Ⅵ) to Cr(Ⅲ). Simultaneously, the solid products (Ti2O(6x−y−z+52)Cl2yCr2x(OH)2z(s)) were in-situ formed and precipitated from the solution due to the continuous generation of hydroxyl ion (OH−) from cathode. This study might provide a new electrochemical method with non-precious metal as the electrode for complete Cr(Ⅵ) removal from aqueous media.

Effects of Complexes and pH Buffer Solution in Electrokinetic Oxidation Treatment on Sediments Chromium Removal

Three electrokinetic (EK) experiments were designed to study the remediation efficiency of Cr(III) by EK-oxidation method and to investigate the influence of complexes and pH buffer solution in EK-oxidation process. Sediments Cr (III) can be oxidized into Cr(VI) effectively by KMnO4, but the oxidation efficiency is not proportional to the dose of oxidant, and chemical oxidation process leads to a bit lower buffering capacity of the sediment. Compared with the simple EK remediation, the removal efficiency of total Cr in the sediments increased 32.6% by EK-oxidation method, and the concentrations of total chromium in the sediment showed increasing trend from the cathode to the anode after EK-oxidation treatment. The pH control and KMnO4 improved the removal of Cr(VI) and total Cr from the sediments. Due to stronger complexation action of citric acid with Cr(III) in sediments, citric acid in the catholyte obviously increased the removal rate of total Cr in sediments. Finally, the removal rate of total Cr from sediments reached 60.9% by adding complex and controlling the pH of cathode and anode pond solution on EK-oxidation processes.

Exchange electrode-electrokinetic remediation of Cr-contaminated soil using solar energy

Abstract The present work focused on analyzing the effects of solar energy on the electrokinetic remediation (EKR) of Cr-contaminated soil, compared to direct current (DC) power supply. Meanwhile, to control the remedied soil pH and focusing phenomena, the exchange electrode-EKR (EE-EKR) has also been employed. Four runs of tests were conducted using DC power supply with the conventional EKR (run A, run D), DC power supply with EE-EKR (run B) and solar energy with EE-EKR (run C) respectively. Important electrokinetic parameters such as electrical voltage and current, electroosmotic flow, soil pH, the remediation efficiency of total Cr, Cr(VI), Cr(III) and cost analysis were investigated to evaluate EE-EKR using solar energy. The results show that run B had the highest removal efficiency of total Cr (43.65%), Cr(VI) (91.88%), and Cr(III) (19.32%), and run C, run A, run D follow in turn. It was proved that EE-EKR could increase the removal efficiency of Cr, especially Cr(III) from contaminated soil, because it could reduce focusing phenomena and enhance electrical current. Meanwhile, the experimental results also indicated that the combination of electrokinetics and solar energy was feasible and effective to some extent for the remediation of Cr-contaminated soil.

Efficient removal of chromium from water by Mn3O4@ZnO/Mn3O4 composite under simulated sunlight irradiation: Synergy of photocatalytic reduction and adsorption

A novel Mn3O4@ZnO/Mn3O4 composite was designed to remove Cr (VI) and Cr (III) from water by concurrent photocatalysis and adsorption. The Mn3O4@ZnO photocatalyst with core-shell nano-sphere structure, was first fabricated by precisely controlled process using a mild hydrothermal method combined with atomic layer deposition (ALD). The wurtzite ZnO layer was uniformly deposited on hausmannite Mn3O4 surface, which formed a typical II-type heterojunction, separating the photo-generated electron-hole pairs effectively. Hence, rapid Cr (VI) reduction by Mn3O4@ZnO under simulated sunlight irradiation was achieved at 95.3% in 110min. The pure Mn3O4 as adsorbent could subsequently adsorb the reduced Cr (III) from aqueous solution. Noticeably, the total Cr removal efficiency was enhanced to 92.0% within 70min by concurrent photocatalysis and adsorption compared to 88.8% even within 120min in the separate two processes. Mechanism exploration proposed that more active sites on Mn3O4@ZnO surface would be released for continuous photocatalytic reduction of Cr (VI) after the transfer of reduced Cr (III) from Mn3O4@ZnO onto Mn3O4 surface. In addition, the synergy of photocatalysis and adsorption on Cr removal was the strongest at pH=6.0 and the highest removal efficiency of 96.0% was realized within 70min. The outstanding synergistic effect of photocatalytic reduction and adsorption makes the innovative Mn3O4@ZnO/Mn3O4 composite a promising candidate for Cr remediation in aquatic environment.

Continuous removal of Cr(VI) from wastewater by phytoextraction using Tradescantia pallida plant based vertical subsurface flow constructed wetland system

This study investigated the performance of Tradescantia pallida in continuous removal of Cr(VI) from contaminated water using a laboratory scale vertical subsurface flow (VSSF) constructed wetland system. The effect of different hydraulic residence time (HRT), i.e. 1, 2 and 3 day HRT, was first studied at an influent Cr(VI) concentration of 20 mg/L. Later experiments were carried out at different influent pH (7 and 4) and at different influent Cr(VI) concentration (20 and 30 mg/L), but with the same HRT of two days. Best results were achieved for 2 d HRT and for an influent pH of 7 with a maximum Cr(VI) removal efficiency of 97.2–98.3% and a maximum total Cr removal efficiency of 86–88.2%. Cr(VI) was found to be uniformly distributed in different segments of the wetland units. Compared with the control unit in which no plants were grown, the units with the plant biomass showed substantially high amount of Cr(VI) in its soil (63–68%). Bioconcentration factor and translocation factor values ranged from 16.45 to 23.21 and 0.37 to 0.90 respectively, which revealed the excellent Cr uptake potential of T. pallida. The highest accumulated Cr concentration (464.33 mg/kg) was found in plants grown in the system at pH 7.0, whereas the minimum accumulated Cr concentration (249.21 mg Cr/kg) was in plants grown in the system at pH 4.0. Furthermore, 3–4% of the total Cr input into the system was accumulated in the plants at the end of the operation. All these results revealed an excellent performance of the T. pallida plant based constructed wetland system for continuous removal of Cr(VI) from contaminated water.

Moisture content-affected electrokinetic remediation of Cr(VI)-contaminated clay by a hydrocalumite barrier.

An electrokinetic-permeable reaction barrier (EK-PRB) system was introduced in this study with hydrocalumite as the barrier material. The combined system effectively remediated the Cr(VI)-contaminated clay after a 72-h treatment, and the Cr(VI) removal efficiency increased with the initial soil moisture content. Further evidence was found that the changing soil pH value and current density were highly associated with the initial moisture content, showing its important roles in the Cr(VI) removal process. Additionally, the total Cr removal efficiency was much lower than that of Cr(VI) owing to the partial conversion of Cr(VI) to Cr(III) in the electrokinetic remediation process. Under high soil moisture conditions (40%), the removal efficiency of Cr(VI) and total Cr was 96.6 and 67.3%, respectively. Further analysis also revealed the new mineral phase, chromate hydrocalumite, for Cr fixation in the hydrocalumite barrier, which was significantly affected by the initial soil moisture content. Our results showed that the EK-PRB system with a hydrocalumite barrier is highly promising with great potential for the effective remediation of Cr(VI)-contaminated clay and engineering implementation.

Effects of reducing agent and approaching anodes on chromium removal in electrokinetic soil remediation

A soil remediation method combining in situ reduction of Cr(VI) with approaching anodes electrokinetic (AAs-EK) remediation is proposed. EK experiments were conducted to compare the effect of approaching anodes (AAs) and fixed electrodes (FEs) with and without sodium bisulfite (NaHSO3) as a reducing agent. When NaHSO3 was added to the soil before EK treatment, 90.3% of the Cr(VI) was reduced to Cr(III). EK experiments showed that the adverse effect of contrasting migration of Cr(III) and Cr(VI) species, which limits the practical application of this technique, was eliminated in the presence of the reducing agent. Furthermore, Tessier fractionation analysis indicated that the reducing agent changed the distribution of the chemical forms of Cr. The AAs-EK method was shown to acidize the soil as the anode moved toward the cathode and this acid front pushed the “focusing” region toward the cathode. After remediation, the pH of the soil was between 1.8 and 5.0 in AAs-EK experiments. The total Cr removal efficiency was 64.4% (except in the “focusing” region) when the reduction reaction was combined with AAs-EK method. We conclude that AAs-EK remediation in the presence of NaHSO3 is an appropriate method for Cr-contaminated soil.

An investigation on reuse of Cr-contaminated sediment: Cr removal and interaction between Cr and organic matter

Sewage sludge, mine tailing and dredged sediment, as well as wastewater rich in heavy metal, pose threats to biota life and their treatments are thorny issues. A novel strategy using Cr-contaminated sediment to treat wastewater containing heavy metals is proposed in this study. The Cr-contaminated sediment (Cr-CS) is used to remove hexavalent chromium (Cr(VI)) from wastewater. Experimental results show that the background Cr concentration in the sediment has little influence on the removal efficiency of total Cr under the examined conditions (300–1,000mgkg−1). The existence of organic matter favors the reduction of Cr(VI), but has limited effect on the removal of Cr(III). The total Cr removal process involves reduction, adsorption and precipitation, and is highly pH-dependent. The optimum pH values for Cr(VI) reduction and Cr(III) removal are determined as 1.0 and 4.0, respectively. The removal efficiency of total Cr under optimum conditions reaches 95.8%. The results also suggested co-existent heavy metals (Cd, Cu, Pb, and Ni) were hard to release to the water under experimental conditions (i.e., the release amounts are ranged from 0.0035 to 0.055mgL−1 for Cd, Cu, Pb, and Ni, respectively, which can achieve the comprehensive sewage discharge standard). This work demonstrated that use of some heavy-metal-contaminated sediments to treat wastewater containing the same heavy metal could be a feasible approach.

Optimum condition for the removal of Cr(VI) or total Cr using dried leaves of Pinus densiflora

A novel biomass, dried leaves of Pinus densiflora, showed very good performance in removing mutagenic Cr(VI) from aqueous solution in this study. Analytical results for different chromium species in aqueous and solid phases indicated that the mechanism of Cr(VI) removal by the biomass was ‘adsorption-coupled reduction’. Among various parameters of batch operation, pH, temperature and contact time significantly affected Cr(VI) removal and total Cr removal individually. As a result, optimal condition for the Cr(VI) removal was not equal to that for the total Cr removal. To examine the individual effects of these parameters on the Cr(VI) and total Cr removals statistically, therefore Box–Behnken model for experiment design and second-order polynomial model for fitting experimental data were used in this study. The removal efficiency of Cr(VI) increased with a decrease in pH or with increases of temperature and contact time until equilibrium had been attained. Meanwhile, an optimum pH existed for the total Cr removal efficiency, but increased with the increases of temperature and contact time. With 60h of contact time, 100% of Cr(VI) removal and >95% of total Cr removal could be obtained at pH 4 and 40°C.

Development of a new Cr(VI)-biosorbent from agricultural biowaste

Among useless but abundant agricultural biowastes such as banana skin, green tea waste, oak leaf, walnut shell, peanut shell and rice husk, in this study, banana skin was screened as the most efficient biomaterial to remove toxic Cr(VI) from aqueous solution. X-ray photoelectron spectroscopy (XPS) study revealed that the mechanism of Cr(VI) biosorption by banana skin was its complete reduction into Cr(III) in both aqueous and solid phases and partial binding of the reduced-Cr(III), in the range of pH 1.5–4 tested. One gram of banana skin could reduce 249.6 (±4.2) mg of Cr(VI) at initial pH 1.5. Namely, Cr(VI)-reducing capacity of banana skin was four times higher than that of a common chemical Cr(VI)-reductant, FeSO 4 · 7H 2O. To diminish undesirable/serious organic leaching from the biomaterial and to enhance removal efficiency of total Cr, its powder was immobilized within Ca-alginate bead. The developed Cr(VI)-biosorbent could completely reduce toxic Cr(VI) to less toxic Cr(III) and could remove almost of the reduced-Cr(III) from aqueous phase. On the basis of removal mechanisms of Cr(VI) and total Cr by the Cr(VI)-biosorbent, a kinetic model was derived and could be successfully used to predict their removal behaviors in aqueous phase. In conclusion, our Cr(VI)-biosorbent must be a potent candidate to substitute for chemical reductants as well as adsorbents for treating Cr(VI)-bearing wastewaters.

Studies on hexavalent chromium biosorption by chemically-treated biomass of Ecklonia sp.

The biomass of the brown seaweed, Ecklonia sp., is capable of reducing Cr(VI) to Cr(III). However, very little is known about the mechanism of Cr(VI) reduction by the biomass. The aims of the present investigation were to enhance the Cr(VI)-reducing capacity of the biomass using various chemical treatments and to elucidate the mechanisms governing Cr(VI) reduction. Among the various chemical treatments, acid-treatment showed the best performance with regards the improvement of Cr(VI) removal from the aqueous phase, while organic solvent-treatment significantly improved the removal efficiency of total Cr in the equilibrium state. Based on FTIR study, the biomass was subjected to chemical modification of its amino and carboxyl groups, to examine their roles in the Cr(VI) removal from the aqueous phase. Methylation of the amino group significantly decreased the Cr(VI) removal rate, but amination of the carboxyl group significantly increased the Cr(VI) removal rate. Meanwhile, esterification of the carboxyl group and carboxylation of the amino group decreased the Cr(VI) removal rate, but the former showed a more negative effect than the latter. These findings indicated that the amino and carboxyl groups take part in the Cr(VI) removal from the aqueous phase. In conclusion, mechanisms for direct and indirect Cr(VI) removal are proposed, and some aspects for the application of this biomass to Cr(VI) detoxification are discussed.

Preliminary Study on Treatment of Soil Enriched in Chromite Ore Processing Residue by Electrokinetics

The effectiveness of the electrokinetic (EK) process in treating soil enriched in chromite ore processing residue (Cr–soil) was investigated. An electric gradient of 1.2 V/cm was applied to induce the movement of electrolyte solution in the soil cell for 17–43 days. Simulated rainwater was used as EK processing fluid. Results show low removal efficiency of total Cr (2–7%) because the insoluble form of chromite dominates in the Cr–soil. However, more than 95% of leachable Cr(VI) can be removed by controlling the pH of the processing fluid at 9.0 during experiments. The removal of Cr from Cr–soil was characterized by the high Cr(VI) concentration collected at the anode, and the presence of Cr precipitates collected at cathode. The electric energy requirement under all experimental conditions varied from 1.3 to 5.8 kW h/m3. Results indicated that the EK is an effective process in mitigating the leaching of Cr(VI) from Cr–soil.